what is user agent?
the http header a client sends to describe itself, naming a browser version and operating system.
it is free text and anyone can set it, so on its own it proves nothing. it is still the first thing checked, because inconsistency is cheap to detect. a chrome user agent arriving from a python client that sends four headers in an unusual order, omits accept-language, and presents a python tls handshake, is an obvious bot before anyone looks harder. it also changes what you receive: sites serve different markup, different layouts and sometimes a stripped mobile page depending on the value, so the wrong string quietly breaks your parser. what works: use a current string from a real browser, keep the rest of the header set consistent with it (accept, accept-language, accept-encoding, sec-ch-ua, and their order), match the platform you claim, and refresh it as browsers release. do not randomise per request. one stable identity per exit is less suspicious than an address that presents ten different browsers in a minute.